Controlling melt-solid interface shape of a growing silicon crystal using a variable magnetic field

ABSTRACT

System for controlling crystal growth in a Czochralski crystal growing apparatus. A magnetic field is applied within the crystal growing apparatus and varied to control a shape of the melt-solid interface where the ingot is being pulled from the melt. The shape of the melt-solid interface is formed to a desired shape in response to the varied magnetic field as a function of a length of the ingot.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.11/026,780, filed Dec. 30, 2004, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

Embodiments of the present invention relate to the field of producingsingle crystal silicon used in the manufacture of electronic components.In particular, embodiments of the invention relate to controlling themelt-solid interface shape of a growing crystal by applying a variablemagnetic field.

BACKGROUND OF THE INVENTION

Single crystal silicon, which is the starting material in most processesfor fabricating semiconductor electronic components, is commonlyprepared according to the so-called Czochralski process. In thisprocess, polycrystalline silicon, or polysilicon, is charged to acrucible and melted, a seed crystal is brought into contact with themolten silicon, and a single crystal ingot is grown by relatively slowextraction. After formation of a neck is complete, decreasing thepulling rate and/or the melt temperature enlarges the diameter of thecrystal until a desired or target diameter is reached. The generallycylindrical main body of the crystal, which has an approximatelyconstant diameter, is then grown by controlling the pull rate and themelt temperature while compensating for the decreasing melt level. Nearthe end of the growth process but before the crucible is emptied ofmolten silicon, the crystal diameter is gradually reduced to form anend-cone. Typically, increasing the crystal pull rate and heat suppliedto the crucible forms the end-cone. When the diameter becomes smallenough, the crystal is then separated from the melt.

As in known in the art, molten silicon (at about 1420 degrees Celsius (°C.)) will dissolve the surface of a silica (SiO₂) crucible containingthe melt. Some of the dissolved silica evaporates from the surface ofthe melt as SiO (silicon monoxide) while some of the dissolved silicabecomes incorporated into the growing crystal. The remainder of thedissolved silica remains in the melt. In this manner, the cruciblecontaining the silicon melt acts as a source of oxygen that is found insilicon crystals grown by the conventional Czochralski technique.

Oxygen in the silicon crystal may have both favorable and unfavorableeffects. In the various heat treatment processes during the manufactureof various electrical devices, the oxygen in the crystal may causecrystal defects such as precipitates, dislocation loops, and stackingfaults or it may cause electrically active defects resulting in deviceswith inferior performance characteristics. The solid solution of oxygenin the crystal, however, increases the mechanical strength of siliconwafers and the crystal defects may improve the yield of conformingproducts by entrapping contaminants of heavy metals. Accordingly, oxygencontent of the silicon crystal is an important factor for productquality that should be carefully controlled in accordance with theultimate application for the silicon wafers.

The oxygen concentration in a conventional silicon crystal grown underCzochralski conditions prevalent in the industry varies along the lengthof the crystal. For example, the concentration is typically higher atthe seed end than in the middle and/or at the bottom or tang end of thecrystal. In addition, oxygen concentration typically varies along theradius of a cross-sectional slice of the crystal.

To address this oxygen control problem, attention has been given to theuse of magnetic fields to stabilize convective flows in metal andsemiconductor melts for controlling oxygen concentration and radialdistribution to remove dopant striation, etc. For example, the Lorentzforce generated by magnetic fields in a conductive melt may be used todampen natural convective flow and turbulence. There are threeconventional types of magnetic field configurations used to stabilizeconvective flows in conductive melts, namely, axial, horizontal, andcusped.

The axial (or vertical) magnetic field configuration (e.g., see FIG. 1)generates a magnetic field parallel to the crystal-growth direction. InFIG. 1, a magnet coil 21, shown in cross-section, supplies a magneticfield to a crucible 23. As shown, the crucible 23 contains a siliconmelt 25 from which a crystal 27 is grown. This configuration has theadvantages of relatively simple setup and axial symmetry. But the axialmagnetic field configuration destroys radial uniformity due to itsdominant axial field component.

In the horizontal (or transverse) magnetic field configuration (e.g.,see FIG. 2), two magnetic poles 29 are placed in opposition to generatea magnetic field perpendicular to the crystal-growth direction. Thehorizontal configuration has the advantage of efficiency in damping aconvective flow at the melt surface. But its non-uniformity both axiallyand radially and the complex and bulky setup make it difficult to applythe horizontal magnetic field configuration in large diameterCzochralski growth processes.

The cusped magnetic field configuration (e.g., see FIG. 3) is designedto overcome the deficiencies of the axial and horizontal magnetic fieldconfigurations. A pair of coils 31, 33 (e.g., Helmholtz coils) placedcoaxially above and below a melt-solid interface and operated in anopposed current mode generates a magnetic field that has a purely radialfield component near the melt surface and a purely axial field componentnear the center of the melt. In this manner, the cusped magnetic fieldconfiguration attempts to preserve the rotational symmetry at theinterface between the melt and the crystal. However, the effectivenessof the cusped magnetic field is decreased at the center of the melt.Furthermore, since the cusp position remains at the melt surface, axialuniformity within the melt is gradually reduced and eventuallydisappears as the melt depth is decreased towards the end of crystalgrowth.

Accordingly, improved control of the crystal growth process is desiredto address the disadvantages of these conventional magnetic fieldconfigurations.

SUMMARY OF THE INVENTION

Embodiments of the invention overcome one or more deficiencies in theprior art and provide control of an interface shape between a melt and acrystal by applying a variable magnetic field. In one embodiment, theinvention controls flow within a silicon melt from which a crystal isgrowing via an asymmetric magnetic field having a configuration andfield intensity that may be continuously changed. Embodiments of theinvention thus allow manipulation of the crystal-to-melt interfaceshape, the axial temperature gradient at the interface, and the radialvariation of the axial temperature gradient in the crystal at and nearthe interface to achieve a desired interface shape and value. Accordingto aspects of the invention, a variable asymmetric magnetic fieldachieves more flexibility and capability than any of the conventionalmagnetic field configurations in controlling the crystal-to-meltinterface shape. Moreover, an embodiment of the invention allows a cuspposition of the magnetic field to be moved either above or below thecrystal-to-melt interface based on a length of the crystal to maintain adesired melt flow control and uniformity. Without changing hardwaresetups and physical locations, the variable asymmetric magnetic field ofthe invention combines the benefits of the conventional magnetic fieldconfigurations while avoiding their deficiencies.

A method embodying aspects of the invention controls crystal growth in acrystal growing apparatus. The method includes applying a cuspedmagnetic field to a semiconductor melt from which a monocrystallineingot is grown according to a Czochralski process. The method alsoincludes varying the magnetic field while the ingot is being pulled fromthe melt to control cusp position of the magnetic field relative to amelt-solid interface between the melt and the ingot for producing adesired shape of the melt-solid interface. The desired shape of themelt-solid interface is a function of length of the ingot.

In another embodiment, a system for controlling crystal growth in acrystal growing apparatus includes first and second coils positionednear a crucible containing a semiconductor melt from which amonocrystalline ingot is grown according to a Czochralski process. Thecoils apply a cusped magnetic field to the melt. The system alsoincludes a variable power supply for energizing the coils. A controllervaries the power supply while the ingot is being pulled from the melt.The variable power supply is responsive to the controller for varyingthe magnetic field to control cusp position of the magnetic fieldrelative to a melt-solid interface between the melt and the ingot. Thisproduces a desired shape of the melt-solid interface. The desired shapeof the melt-solid interface is a function of length of the ingot.

Yet another embodiment involves a method for producing a monocrystallinesemiconductor ingot by a Czochralski process. The method includesgrowing a monocrystalline ingot on a seed crystal pulled from asemiconductor melt. The method also includes applying an asymmetricmagnetic field to the melt while growing the ingot and varying themagnetic field, as a function of a length of the ingot, to control ashape of the melt-solid interface while the ingot is being pulled fromthe melt.

Another method embodying aspects of the invention controls an oxygencharacteristic of crystal growth of a monocrystalline ingot. The ingotis grown in a crystal growing apparatus according to a Czochralskiprocess. The crystal growing apparatus has a heated crucible thatincludes a semiconductor melt from which the ingot is grown. The ingotis grown on a seed crystal pulled from the melt. The method includesapplying a cusped magnetic field to the melt. The method also includesvarying the magnetic field while the ingot is being pulled from the meltto control cusp position of the magnetic field relative to a melt-solidinterface between the melt and the ingot for producing a desired shapeof the melt-solid interface. The desired shape of the melt-solidinterface produces a desired oxygen characteristic in the ingot.

Alternatively, embodiments of the invention may comprise various othermethods and apparatuses.

Other features will be in part apparent and in part pointed outhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating an axial magnetic field to acrystal growing apparatus according to the prior art.

FIG. 2 is a block diagram illustrating a horizontal magnetic field to acrystal growing apparatus according to the prior art.

FIG. 3 is a block diagram illustrating a cusped magnetic field to acrystal growing apparatus according to the prior art.

FIG. 4 is an illustration of a crystal growing apparatus and anapparatus according to an embodiment of the present invention forcontrolling the crystal growing apparatus.

FIG. 5A is an illustration of an exemplary melt-solid interface having agenerally convex shape relative to an ingot.

FIG. 5B is an illustration of an exemplary melt-solid interface having agenerally concave shape relative to an ingot.

FIG. 5C is an illustration of an exemplary melt-solid interface having agenerally gull-wing shape.

FIG. 6A is a block diagram illustrating a horizontally dominatedasymmetric magnetic field according to one embodiment of the invention.

FIG. 6B is an exemplary graph illustrating a power distribution in ahorizontally dominated asymmetric magnetic field according to oneembodiment of the invention.

FIG. 6C is an exemplary graph illustrating melt-solid interface shapesgenerated by a horizontally dominated asymmetric field according to oneembodiment of the invention compared to melt-solid interface shapesgenerated by a conventional cusped magnetic field.

FIG. 7A is a block diagram illustrating an axially dominated asymmetricmagnetic field according to one embodiment of the invention.

FIG. 7B is an exemplary graph illustrating a power distribution in anaxially dominated asymmetric magnetic field according to one embodimentof the invention.

FIG. 7C is an exemplary graph illustrating melt-solid interface shapesgenerated by an axially dominated asymmetric field according to oneembodiment of the invention compared to melt-solid interface shapesgenerated by a conventional cusped magnetic field.

FIG. 7D is an exemplary graph illustrating oxygen concentrationsgenerated by an axially dominated magnetic field according to oneembodiment of the invention compared to oxygen concentrations generatedby a conventional cusped magnetic field as a function of crystal length.

FIG. 7E is an exemplary graph illustrating oxygen concentrationsgenerated by a horizontally dominated magnetic field according to oneembodiment of the invention compared to oxygen concentrations generatedby a conventional cusped magnetic field as a function of crystal length.

FIG. 8 is a block diagram illustrating a symmetric magnetic fieldaccording to one embodiment of the invention.

FIG. 9 is an exemplary graph illustrating changing oxygen radialgradients as a function of changes in crystal length when a cuspposition is above or below a melt surface and when the cusp position isnear the melt surface according to one embodiment of the invention.

FIG. 10A is an exemplary graph illustrating a melt-solid interfacecomparison between a standard silicon growth process and cruciblerotation modulation according to one embodiment of the invention.

FIG. 10B is an exemplary graph illustrating interface gradients of acrucible rotation modulation according to one embodiment of theinvention compared to interface gradients of a conventional silicongrowth process.

FIG. 10C is an exemplary graph illustrating interface v/G_(s) of acrucible rotation modulation according to one embodiment of theinvention compared to interface v/G_(s) of a conventional silicon growthprocess.

FIG. 11 is an exemplary flow diagram illustrating process flow accordingto one embodiment of the invention for use in combination with a crystalgrowing apparatus for growing a monocrystalline ingot according to aCzochralski process.

Corresponding reference characters indicate corresponding partsthroughout the drawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 4, a Czochralski crystal growth apparatusembodying aspects of the present invention is shown in block diagramform. In general, the crystal growing apparatus includes a vacuumchamber 101 enclosing a crucible 103. Heating means such as a resistanceheater 105 surrounds the crucible 103. During heating and crystalpulling, a crucible drive unit (e.g., a motor) 107 rotates crucible 103,for example, in the clockwise direction as indicated by the arrow. Thecrucible drive unit 107 may also raise and/or lower crucible 103 asdesired during the growth process. Within crucible 103 is a silicon melt109 having a melt level 111. In operation, the apparatus pulls a singlecrystal 113, starting with a seed crystal 115 attached to a pull shaftor cable 117, from the melt 109. As is known in the art, one end of thepull shaft or cable 117 is connected by way of a pulley (not shown) to adrum (not shown), and the other end is connected to a chuck (not shown)that holds the seed crystal 115 and the crystal 113 grown from seedcrystal 115.

The crucible 103 and single crystal 113 have a common axis of symmetry119. Crucible drive unit 107 can raise crucible 103 along the axis 119as the melt 109 is depleted to maintain its level 111 at a desiredheight. A crystal drive unit 121 similarly rotates pull shaft or cable117 in a direction opposite the direction in which crucible drive unit107 rotates crucible 103 (e.g., counter-rotation). In embodiments usingiso-rotation, the crystal drive unit 121 may rotate pull shaft or cable117 in the same direction in which crucible drive unit 107 rotatescrucible 103 (e.g., in the clockwise direction). In addition, crystaldrive unit 121 raises and lowers the crystal 113 relative to melt level111 as desired during the growth process.

According to the Czochralski single crystal growth process, a quantityof polycrystalline silicon, or polysilicon, is charged to crucible 103.A heater power supply 123 energizes the resistance heater 105, andinsulation 125 lines the inner wall of the vacuum chamber 101. A gassupply (e.g., a bottle) 127 feeds argon gas to vacuum chamber 101 via agas flow controller 129 as a vacuum pump 131 removes gas from vacuumchamber 101. A chamber cooling jacket 133, which is fed with coolingwater from a reservoir 135, surrounds vacuum chamber. The cooling wateris then drained to a cooling water return manifold 137. Typically, atemperature sensor such as a photocell 139 (or pyrometer) measures thetemperature of melt 109 at its surface, and a diameter transducer 141measures the diameter of single crystal 113. A processor such as acontrol unit 143 processes the signals generated by the photocell 139and the diameter transducer 141. The control unit 143 may be aprogrammed digital or analog computer; it controls crucible and singlecrystal drive units 107 and 121, heater power supply 123, pump 131, andargon flow controller 129.

As shown in FIG. 4, an upper magnet, such as a solenoid coil 145, and alower magnet, such as a solenoid coil 147, may be located above andbelow, respectively, melt level 111. In the illustrated embodiment, thecoils 145, 147, shown in cross-section, surround vacuum chamber 101 andshare axes with axis of symmetry 119. The upper and lower coils 145, 147have separate power supplies, namely, an upper coil power supply 149 anda lower coil power supply 151, each of which is connected to andcontrolled by control unit 143.

According to embodiments of the invention, current flows in oppositedirections in the two solenoid coils 145, 147 to produce a magneticfield. A reservoir 153 provides cooling water to the upper and lowercoils 145, 147 before draining via cooling water return manifold 137. Aferrous shield 155 surrounds coils 145, 147 in the illustratedembodiment to reduce stray magnetic field and to enhance the strength ofthe field produced.

Embodiments of the invention involve producing silicon crystal ingotssuitable for use in device manufacturing. Advantageously, the presentinvention may be used to produce silicon crystal 113, a substantialportion or all of which is substantially free of agglomerated intrinsicpoint defects. That is, a substantial portion or all of crystal 113produced by embodiments of the invention may have a density of defectsof less than about 1×10⁴ defects/cm³, less than about 5×10³ defects/cm³,less than about 1×10³ defects/cm³, or even no detectable agglomeratedintrinsic point defects. In further embodiments, the present inventionmay be used to produce crystal 113 having substantially no agglomerateddefects that are larger than about 60 nm in diameter.

Aspects of the present invention control the shape of the melt-solid ormelt-crystal interface during crystal growth to limit and/or suppressthe formation of agglomerated intrinsic point defects. FIG. 5 shows anexemplary melt-crystal interface, including melt surface 161. The shapeof this interface between melt 109 and silicon crystal 113 may beconcave or convex in shape relative to the crystal 113. The melt-solidinterface shape may even be both concave and convex in shape relative tocrystal 113 (e.g., a “gull-wing” shape). As described below, themelt-solid interface shape is an important parameter for controllingdefects during crystal growth.

In one embodiment, the present invention employs melt convection toaffect the melt-solid interface shape. Convection refers to the processof heat transfer in a liquid by the movement of the liquid itself. Ingeneral, there are two types of convection: natural convention andforced convection. Natural convection occurs when the movement of melt109 is due, for example, to the presence of heaters 105 giving rise todensity gradients. Forced convection occurs when the movement of themelt 109 is due to an external agent such as a magnetic field incrucible 103. Accordingly, controlling the magnetic field intensity mayproduce a desired melt-solid interface shape.

For instance, because a magnetic field may affect the flow pattern inelectrically conducting fluids such as silicon melt 109, an embodimentof the invention uses a magnet (e.g., in the form of coils 145, 147) toaffect melt convection and thus change the temperature distribution inthe melt 109, which in turn affects the melt-solid interface shape. Asdescribed below, embodiments of the invention further control the flowof silicon melt 109 via an axially asymmetric magnetic field whoseconfiguration and field intensity may be continuously changed. As such,the melt-solid interface shape, the axial temperature gradient at theinterface, and the radial variation of the axial temperature gradient incrystal 113 at and near the interface may be manipulated to produce adesired interface shape and value. As described in detail below withreference to FIG. 6A, FIG. 7A, and FIG. 8, embodiments of the presentinvention provide variable magnetic fields within the crystal growingapparatus for producing desired melt-solid interface shapes, which inturn produce desired crystal characteristics.

As is known to those skilled in the art, a silicon crystal grown from amelt may have an excess of crystal lattice vacancies (“V”) or siliconself-interstitials (“I”). According to embodiments of the presentinvention, manipulation of a melt-solid interface shape during thegrowth of the crystal can be used to control the initial distribution ofpoint defects at the solidification front and the diffusion path ofpoint defects at various degrees from the melt-solid interface. Thedominant point defect type is generally determined near solidification.Therefore, if the dominant point defect concentration reaches a level ofcritical super-saturation and if the mobility of the point defect issufficiently high, a reaction or an agglomeration event is likely tooccur. Agglomerated intrinsic point defects in silicon may affect theyield potential of the material in the production of complex and highlyintegrated circuits. By controlling the melt-solid interface shape,embodiments of the invention reduce or avoid the agglomeration reactionto produce silicon that is substantially free of agglomerated intrinsicpoint defects.

The ratio of the pull rate v of a crystal to an axial temperaturegradient G indicates the type of intrinsic point defect likely to occurin the growing crystal. For example, when the pull rate is high, latticevacancies are generally the dominant point defects. Alternatively, whenthe pull rate is low, silicon self-interstitials are generally thedominant point defects. Thus, during a dynamic growth process (i.e.,where v/G may vary as a function of the radius and/or axial length ofthe crystal), point defects within the growing silicon crystal maychange from being vacancy dominated to interstitial dominated, or viceversa. Moreover, an identifiable vacancies/self-interstitials (V/I)transition is associated with such a change. A defect free region mayexist between agglomerated vacancy defects and agglomerated interstitialtype defects. Specifically, the V/I transition occurs within thisdefect-free region. That is, this defect-free region corresponds to thetransition region from an excess vacancy dominant region to an excessinterstitial dominant region. The defect-free region may be vacancydominated and/or interstitial dominated material. The defect-free regiondoes not include critical excess point defects to form any defects andgenerally includes the V/I transition.

When the identified V/I transition has a particular shape as providedherein, crystal 113 is substantially free of agglomerated defects atthis transition. For instance, a substantially flat V/I transitionperpendicular to the pull axis 119 under dynamic growth simulationscorresponds to a portion of crystal 113 substantially free ofagglomerated defects. Therefore, controlling a melt-solid interfaceshape produces improvements in substantially defect-free single siliconcrystal. In particular, by controlling the melt-solid interface shapeaccording to a target melt-solid interface shape as a function of axiallength, embodiments of the invention can produce a region substantiallyfree of agglomerated defects in crystal 113.

The target interface shape is unique to the crystal hot zone design andthe position along the axial length of the crystal 113. Accordingly, thetarget interface shape is determined for a particular hot zone atvarious positions along the length of crystal 113. Rather than limitingthe rate at which such defects form or attempting to annihilate some ofthe defects after they have formed, suppressing or otherwise controllingthe agglomeration reactions yields a silicon substrate that issubstantially free of undesirable amounts or sizes of agglomeratedintrinsic point defects. Suppressing or controlling the agglomerationreactions also affords single crystal silicon wafers having an epi-likeyield potential in terms of the number of integrated circuits obtainedper wafer and without having the high costs associated with an epitaxialprocess.

By affecting the melt-solid interface shape, a magnetic field imposedupon melt 109 in accordance with embodiments of the invention canregulate the oxygen concentration in the axial and radial directions forsingle crystal ingots of relatively large diameter, particularly atrelatively low oxygen concentrations. Current is permeated through theupper and lower coils 145, 147 as indicated (the “•” indicating the flowof current out of the page and the “X” indicating the flow of currentinto the page), thereby causing a magnetic field to be imposed uponcrucible 103 and silicon melt 109. The magnetic field has horizontal andvertical components that perpendicularly intersect the bottom andsidewalls of crucible 103. In addition, the magnetic field may have avertical component that perpendicularly intersects silicon melt surface161. The average magnetic component that perpendicularly intersectsmolten silicon surface 161 may be small relative to the average magneticcomponent perpendicularly intersecting the bottom and sidewalls ofcrucible 103 in contact with melt 109. That is, the average magneticcomponent that perpendicularly intersects melt surface 161 may be nogreater than about one-tenth of the average magnetic componentperpendicularly intersecting the bottom and side walls of crucible 103in contact with the molten silicon of melt 109. Moreover, the averagemagnetic component perpendicularly intersecting melt surface 161 may beat or near zero. That is, the magnetic field null plane is located at ornear silicon melt level 111. Vertical position, the number of turns, andthe relative current in the two coils 145, 147 may position the nullmagnetic field at or near the plane of melt level 111.

Embodiments of the invention provide a variable asymmetric magneticfield configuration (see FIG. 6A and FIG. 7A) that advantageously usesthe same hardware setup as the cusped magnetic field configuration (seeFIG. 3). According to one embodiment, control unit 143 (i.e., acontroller) controls upper and lower coil power supplies 149 and 151 toadjust the power distribution of upper and lower coils 145 and 147 suchthat an axially asymmetric field intensity generated in upper and lowercoils 145 and 147 moves a cusp position to above or below melt level 111(e.g., at a melt-solid interface). Control unit 143 may further controlupper and lower coil power supplies 149 and 151 to impose differentasymmetric field configurations upon the melt-solid interface such thatan axially dominated asymmetric field configuration, horizontallydominated asymmetric field configuration, or symmetric fieldconfiguration (e.g., cusped magnetic field configuration) may beachieved at any crystal length. Therefore, embodiments of the inventionprovide the desired melt flow control and melt flow uniformity at thesame time with improved efficiency, flexibility, and capability thatcombine the benefits from the three conventional magnetic fieldconfigurations while avoiding their deficiencies.

To move the cusp position up and down and to change the degree (ormagnitude) of axial or horizontal field domination, control unit 143adjusts upper and lower coil power supplies 149 and 151 to change thepower distributions in upper and lower coils 145 and 147. In anembodiment of the invention, upper and lower coils 145 and 147 are madesuch that when using the same power distribution (e.g., both at the samepercentage of the maximum power input), the cusp position remains atmelt surface 111 (e.g., the melt-solid interface). Using the maximumpower input typically enables a single coil (e.g., upper coil 145 orlower coil 147) to generate a magnetic field of a few hundreds to acouple of thousand gauss depending on the size of the coil.

According to embodiments of the invention, control unit 143 isconfigured to move the cusp position to above or below melt level 111with variable distance by adjusting the difference of powerdistributions to upper and lower coils 145 and 147 (e.g., via upper andlower coil power supplies 149 and 151). Control unit 143 is alsoconfigured to adjust the variable distance above or below melt level 111as a function of time or crystal length so that the variable cuspposition varies accordingly at different crystal growth stages anddifferent crystal lengths.

In the crystal pulling process, current is permeated through coils 145,147 to impose a magnetic field upon silicon melt 109 and crucible 103having a predetermined intensity. The predetermined intensity variesdepending upon the diameter of crystal 113, the diameter of crucible103, the amount of the polysilicon charge, and the desired oxygencontent. In general, the invention involves a magnetic field having amaximum predetermined intensity of, for example, less than severalthousand gauss such as between about 400 and 2000 gauss. As the lengthof crystal 113 increases (i.e., as the fraction of the molten chargesolidified increases), control unit 143 decreases the intensity of thefield by reducing the amount of current permeated through the coils(e.g., by controlling upper and lower coil power supplies 149 and 151),by moving the coils relative to crucible 103, or by moving oreliminating a magnetic shielding.

As control unit 143 decreases the intensity of the magnetic field, themagnetic field components that perpendicularly intersect the bottom andside walls of crucible 103 is reduced. But because the null plane of themagnetic field remains at or near silicon melt surface 161, the ratiobetween the average magnetic field component that perpendicularlyintersects silicon melt surface 161 and the average magnetic fieldcomponent that perpendicularly intersects the bottom and side walls ofcrucible 103 in contact with the molten silicon of melt 109 may notchange.

Depending upon such parameters as single crystal nominal diameter,crystal length, crucible diameter, charge size, and magnetic fieldcharacteristics, control unit 143 controls upper and lower coil powersupplies 149 and 151 to increase or decrease the strength of themagnetic field imposed on melt 109. For example, control unit 143 maycontrol upper and lower coil power supplies 149 and 151 to adjust theintensity of the magnetic field to some value less than its initiallevel as the length of crystal 113 increases and the fraction of themolten charge solidified increases. Control unit 143 may also completelyturn off the magnetic field after a predetermined fraction of the moltencharge is frozen. In one embodiment, control unit 143 turns off themagnetic field after approximately 50% to 80% of the molten charge isfrozen. Thereafter, control unit 143 may further regulate the oxygencontent by increasing a crucible rotation rate relative to a singlecrystal rotation rate.

In one embodiment of the invention, maintaining the melt-solid interfaceshape within a certain range or percentage of a height deviation ratio(HDR) by controlling magnetic field intensity is desired. The HDR isdetermined from the following equation:HDR=[H _(c) −H _(e)]/Radius X100,where H_(c) is the height of the crystal center from melt level 111 andH_(e) is the height of the crystal edge from melt level 111. Forexample, one embodiment of the invention controls the melt-solidinterface for a 200 millimeter (mm) crystal such that the HDR betweenthe crystal center and the crystal edge is about plus or minus 11%, 9%,7%, or 5%. For crystals having a diameter other than 200 mm, the maximumHDR can be gradually decreased by a slope of about −0.06 by the crystalradius.

Embodiments of the invention may be used to control oxygen concentrationin single crystals having relatively low oxygen concentration (e.g.,less than parts per million atoms (PPMA) oxygen). These single crystalingots may have an oxygen gradient of less than 5% in the radialdirection and less than 5% to 10% in the axial direction.

As a specific example, embodiments of the invention may be utilized toimprove the capabilities of the 200 mm silicon wafer manufacturingprocess. This manufacturing process typically relies on the followingstrategies. First, a crystal are grown at (v/G)_(s) close to but slightlower than (v/G)_(c), which is the critical value of (v/G)_(s), so thatthe crystal is slightly interstitial rich. The temperature of thecrystal is then maintained at above 900° C., which is the nucleationtemperature of interstitial defects, to promote the diffusion and thusannihilation of vacancies and interstitials. After the crystal growth,the crystal is quenched by moving it into an upper chamber to suppressthe defect nucleation and growth. Typically, it is desirable to have ahigher axial temperature gradient at the melt-solid interface so that agood silicon quality may be achieved at a higher pull rate and thus at ahigher throughput. However, due to the limitation of hot zone, a higheraxial temperature gradient may come with a higher radial variation ofthe axial temperature gradient in the crystal at and near the melt-solidinterface. The higher variation of the axial temperature gradient maythen reduce the uniformity of (v/G)_(s) in the crystal. Sometimes,efforts to increase the axial temperature gradient cause the variationto increase and thus generate coexistence of V and I defects.

Embodiments of the invention may be used to solve this problem. Forexample, control unit 143 may apply a higher power distribution in uppercoil 145 than in lower coil 147 such that the asymmetric magnetic fieldmoves the cusp position to below melt level 111 to generate ahorizontally dominated asymmetric magnetic field at melt surface 161, asshown in FIG. 6A. This field condition leads to a less concave-shapedand more gull-wing shaped melt-solid interface, which has a higher axialtemperature gradient and a flatter radial variation of the axialtemperature gradient in crystal 113 at and near the melt-solidinterface. FIG. 6B shows an exemplary graph illustrating the powerdistribution in a horizontally dominated asymmetric magnetic field.Particularly, FIG. 6B illustrates changes in percentage of power inputfor both upper and lower coils 145, 147 as a function of changes incrystal length. FIG. 6C shows an exemplary graph illustrating thedifference between melt-solid interface shapes generated by ahorizontally dominated asymmetric magnetic field and a standard cuspedmagnetic field. As may be seen, the melt-solid interface shape generatedby the horizontally dominated asymmetric magnetic field has beenadjusted toward a desired direction.

To achieve a more stable melt flow, in some growth stages such asnecking, crown, and late-body to end-cone growth, control unit 143 maygenerate an axially dominated asymmetric magnetic field at melt surface161 by applying a higher power distribution in lower coil 147 than inupper coil 145 so that the asymmetric magnetic field moves the cuspposition to above melt level 111, as shown in FIG. 7A. This fieldcondition leads to a more convex-shaped melt-solid interface and a loweroxygen concentration in crystal 113. FIG. 7B shows an exemplary graphillustrating the power distribution in an axially dominated asymmetricmagnetic field. Particularly, FIG. 7B illustrates changes in percentageof power input for both upper and lower coils 145, 147 as a function ofchanges in crystal length. FIG. 7C shows an exemplary graph illustratingthe difference between melt-solid interface shapes generated by anaxially dominated asymmetric magnetic field and a standard cuspedmagnetic field. As may be seen, the melt-solid interface shape generatedby the axially dominated asymmetric magnetic field has been adjustedtoward a desired direction. FIG. 7D shows an exemplary graphillustrating a difference between oxygen concentrations (Oi) generatedby an axially dominated magnetic field and a standard cusped magneticfield as a function of crystal length. FIG. 7E shows an exemplary graphillustrating a difference between oxygen concentrations (Oi) generatedby a horizontally dominated magnetic field and a standard cuspedmagnetic field as a function of crystal length.

In growth stages crystal lengths where the standard cusped magneticfield (e.g., a symmetric magnetic field) is desired, control unit 143may configure this symmetric setting by applying a substantially uniformpower distribution between upper and lower coils 145 and 147 to move thecusp position to near melt level 111, as shown in FIG. 8. FIG. 9 showsan exemplary graph illustrating changing oxygen radial gradients (ORGs)as a function of changes in crystal length when a cusp position is atabove or below melt level 111 (i.e., asymmetric magnetic fields) andwhen the cusp position is at near melt level 111 (i.e., symmetricmagnetic field).

As mentioned earlier, applying variable asymmetric magnetic fields atvarious crystal lengths can generate various desired interface shapesand consequently desired radial and axial temperature gradients as wellas v/G_(s) can be achieved at desired body lengths. FIGS. 6A, 6B and 6Cshow that horizontally dominated asymmetric magnetic field at meltsurface 161 can generate a less concave-shaped and more gull-wing shapedmelt-solid interface. FIG. 10A shows an exemplary graph illustrating themelt-solid interface comparison between the standard interface and amore gull-wing shaped interface at a crystal length of 480 mm. FIG. 10Bshows an exemplary graph illustrating a difference between interfacegradients of the standard interface and a more gull-wing shapedinterface. Particularly, FIG. 10B shows changes in axial temperaturegradients of the standard interface and a more gull-wing shapedinterface as a function of changes in distance from axis on theinterface. FIG. 10C shows an exemplary graph illustrating a differencebetween interface v/G_(s) of the standard interface and a more gull-wingshaped interface. Particularly, FIG. 10C shows changes in v/G_(s) on amelt-solid interface of the standard interface and a more gull-wingshaped interface as a function of changes in distance from axis on theinterface.

Referring to FIG. 11, an exemplary flow diagram illustrates process flowaccording to one embodiment of the invention for use in combination witha crystal growing apparatus for growing a monocrystalline ingotaccording to a Czochralski process. The crystal growing apparatus has aheated crucible including a semiconductor melt from which the ingot ispulled. The ingot is grown on a seed crystal pulled from the melt. Theshape of the melt-crystal interface is formed as a function of a lengthof the ingot. At 1002, an external magnetic field is imposed in avicinity of a melt-solid interface between the melt and the ingot duringpulling of the ingot. For example, the external magnetic field may beimposed by a first magnet (e.g., a first solenoid) situated at above themelt-solid interface and a second magnet (e.g., a second solenoid)situated at below the melt-solid interface.

At 1004, the external magnetic field is selectively adjusted to controla shape of the melt-solid interface while the ingot is being pulled fromthe melt. For example, a horizontally dominated asymmetric magneticfield configuration, an axially dominated asymmetric magnetic fieldconfiguration, or a symmetric magnetic field configuration may beapplied to the vicinity of the melt-solid interface. If the horizontallydominated asymmetric magnetic field configuration is applied,embodiments of the invention achieve a less concave and more gull-wingshaped interface shape relative to the ingot with a flatter and moreconcave interface, a higher axial temperature at the melt-solidinterface, and a lower radial variation of the axial temperaturegradient in the ingot at and near the melt-solid interface. If theaxially dominated asymmetric magnetic field configuration is applied,embodiments of the invention achieve a more convex interface shaperelative to the ingot, a substantially stable melt flow, and a lowerlevel of oxygen concentration in the ingot.

To selectively adjust the external magnetic field, a power distributionof the first magnet and the second magnet is adjust to move a cuspposition (e.g., at a variable distance as a function of the length ofthe ingot and/or a growth stage of the ingot) to above or below themelt-solid interface via a magnetic field intensity generated in thefirst magnet and the second magnet. Thus, a higher power distributionmay be applied in the first magnet than in the second magnet to move thecusp position to below the melt-solid interface to achieve ahorizontally dominated asymmetric magnetic field configuration.Similarly, a higher power distribution may be applied in the secondmagnet than in the first magnet to move the cusp position to above themelt-solid interface to achieve an axially dominated asymmetric magneticfield configuration. Furthermore, a substantially uniform powerdistribution may be applied between the first magnet and the secondmagnet to move the cusp position to near the melt-solid interface toachieve a symmetric magnetic field configuration.

To selectively adjust the external magnetic field, a configuration ofthe external magnetic field and/or an intensity of the external magneticfield may be varied while the ingot is being pulled from the melt andaccording to the length of the ingot and/or the growth stage (e.g.,during necking, crown, or late-body to end-cone growth) of the ingot.The external magnetic field may also be adjusted to control the shape ofthe melt-solid interface to maintain a level of oxygen concentrationand/or a level of oxygen radial gradient in the ingot. The externalmagnetic field may further be adjusted to achieve a desired shape (e.g.,a convex interface shape relative to the ingot, a concave interfaceshape relative to the ingot, or a gull-wing interface shape) of themelt-solid interface at a desired length of the ingot.

The order of execution or performance of the methods illustrated anddescribed herein is not essential, unless otherwise specified. That is,it is contemplated by the inventors that elements of the methods may beperformed in any order, unless otherwise specified, and that the methodsmay include more or less elements than those disclosed herein.

When introducing elements of the present invention or the embodimentsthereof, the articles “a,” “an,” “the,” and “said” are intended to meanthat there are one or more of the elements. The terms “comprising,”“including,” and “having” are intended to be inclusive and mean thatthere may be additional elements other than the listed elements.

1. A system for controlling crystal growth in a crystal growingapparatus, said crystal growing apparatus having a heated crucibleincluding a semiconductor melt from which a monocrystalline ingot isgrown according to a Czochralski process, said ingot being grown on aseed crystal pulled from the melt, said melt and said ingot forming amelt-solid interface therebetween, said system comprising: first andsecond coils positioned near the crucible for applying a cusped magneticfield to the melt; a variable power supply for energizing the coils toproduce the magnetic field applied to the melt; and a controller forvarying the power supply while the ingot is being pulled from the melt,said variable power supply being responsive to the controller forvarying the magnetic field to control cusp position of the magneticfield relative to the melt-solid interface between the melt and theingot to control a shape of the melt-solid interface, said controlledshape of the melt-solid interface being a function of length of theingot.
 2. The system of claim 1 wherein the variable power supply isresponsive to the controller for varying the magnetic field according toone or more of the following types of magnetic field configurationsrelative to the melt-solid interface: a horizontally dominatedasymmetric magnetic field configuration; an axially dominated asymmetricmagnetic field configuration; and a substantially symmetric magneticfield configuration.
 3. The system of claim 2 wherein the horizontallydominated asymmetric magnetic field produces one or more of thefollowing: a melt-solid interface shape having a flatter concavegull-wing shape relative to the ingot; an increased axial temperaturegradient at the melt-solid interface; and a decreased radial variationof the axial temperature gradient in the ingot near the melt-solidinterface.
 4. The system of claim 2 wherein the axially dominatedasymmetric magnetic field produces one or more of the following: amelt-solid interface shape having a more convex shape relative to theingot; a substantially stable melt flow; and a decreased level of oxygenconcentration in the ingot.
 5. The system of claim 1 wherein the firstcoil is positioned higher than the melt-solid interface and the secondcoil is positioned lower than the melt-solid interface.
 6. The system ofclaim 5 wherein the variable power supply is responsive to thecontroller for increasing a power distribution in the first coilrelative to the second coil to move the cusp position below themelt-solid interface for achieving a horizontally dominated asymmetricmagnetic field configuration.
 7. The system of claim 5 wherein thevariable power supply is responsive to the controller for increasing apower distribution in the second coil relative to the first coil to movethe cusp position above the melt-solid interface for achieving anaxially dominated asymmetric magnetic field configuration.
 8. The systemof claim 5 wherein the variable power supply is responsive to thecontroller for energizing the first and second coils according to asubstantially uniform power distribution to move the cusp position nearthe melt-solid interface for achieving a substantially symmetricmagnetic field configuration.
 9. The system of claim 5 wherein thecontroller varies the power supply to selectively adjust a powerdistribution of the first and second coils as a function of one or moreof the following to change the magnetic field intensity thereby movingthe cusp position to a particular position above or below the melt-solidinterface: length of the ingot; and growth stage of the ingot.
 10. Thesystem of claim 9 wherein the growth stage includes one or more of thefollowing: necking; crown; and late-body to end cone growth.
 11. Thesystem of claim 1 wherein the controller is responsive to a desiredlevel of oxygen concentration for varying the power supply toselectively adjust the magnetic field to control the shape of themelt-solid interface to produce the desired level of oxygenconcentration in the ingot.
 12. The system of claim 1 wherein thecontroller is responsive to a desired level of oxygen radial gradientfor varying the power supply to selectively adjust the magnetic field tocontrol the shape of the melt-solid interface to produce the desiredlevel of oxygen radial gradient in the ingot.
 13. The system of claim 1wherein the first and second coils comprise solenoids.
 14. The system ofclaim 1 wherein the variable power supply includes a first coil powersupply for energizing the first coil and a second coil power supply forenergizing the second coil.
 15. A system for controlling crystal growthin a crystal growing apparatus, said crystal growing apparatus having aheated crucible including a semiconductor melt from which amonocrystalline ingot is grown according to a Czochralski process, saidingot being grown on a seed crystal pulled from the melt, said systemcomprising: first and second coils positioned near the crucible forapplying a cusped magnetic field to the melt, said magnetic field beingbased on power distributed to said first and second coils; a first powersupply for supplying the first coil with power according to a firstpower distribution; a second power supply for supplying the second coilwith power according to a second power distribution; and a controllerfor controlling the first and second power supplies while the ingot isbeing pulled from the melt to produce the magnetic field applied to themelt, said controller controlling the first and second power suppliesaccording to the first and second power distributions, respectively, forvarying the magnetic field to control a shape of the melt-interface as afunction of length of the ingot.
 16. The system of claim 15 wherein thecontroller controls the first and second power distributions such thatthe first power distribution is greater than the second powerdistribution to move the cusp position below the melt-solid interfacefor achieving a horizontally dominated asymmetric magnetic fieldconfiguration.
 17. The system of claim 15 wherein the controllercontrols the first and second power distributions such that the secondpower distribution is greater than the first power distribution to movethe cusp position above the melt-solid interface for achieving anaxially dominated asymmetric magnetic field configuration.
 18. Thesystem of claim 15 wherein the controller controls the first and secondpower distributions such that the first and second power distributionsare substantially uniform power to move the cusp position near themelt-solid interface for achieving a substantially symmetric magneticfield configuration.
 19. The system of claim 15 wherein the controllercontrols the first and second power distributions, relative to eachother, as a function of one or more of the following to change themagnetic field intensity thereby moving the cusp position to a desiredposition above or below the melt-solid interface: length of the ingot;and growth stage of the ingot.
 20. A system for producing amonocrystalline semiconductor ingot by a Czochralski process, saidsystem comprising: a crystal growing apparatus having a heated crucible,said crucible containing a semiconductor melt from which the ingot isgrown on a seed crystal pulled from the melt, said melt and said ingotforming a melt-solid interface therebetween; first and second coilspositioned near the crucible for applying a cusped magnetic field to themelt; a variable power supply for energizing the coils to produce themagnetic field applied to the melt; and a controller for varying thepower supply while the ingot is being pulled from the melt, saidvariable power supply being responsive to the controller for varying themagnetic field to control cusp position of the magnetic field relativeto the melt-solid interface between the melt and the ingot forcontrolling a shape of the melt-solid interface, said controlled shapeof the melt-solid interface being a function of length of the ingot.